This project is concerned with the preparation of fresh human platelet concentrates in connection with clinical research programs which include intensive chemotherapy and/or radiotherapy as a part of the total care of patients with acute leukemia or other neoplastic disease. Platelets will be collected by plasmapheresis and prepared as unmodified concentrates. Clinical studies will include evaluation of HL-A histocompatability, and the hemostatic effectiveness of low-dose platelet transfusions. Other studies will be directed to improvements in platelet preparation and preservation, with particular respect to the clinical effectiveness of platelet concentrates following short-term storage under various conditions. Parallel studies will be undertaken on the collection, preservation, and evaluation of normal human granulocyte concentrates prepared by "filtration leukopheresis".